In the fabrication of semiconductor devices from a silicon wafer, a variety of semiconductor processing equipment and tools are utilized. One of these processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shadow trench isolation (STI) layer, on an inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer which are frequently used in memory devices. The planarization process is important since it enables the use of a high resolution lithographic process to fabricate the next level circuit. The accuracy of a high resolution lithographic process can be achieved only when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of semiconductor devices.
A global planarization process can be carried out by a technique known as chemical mechanical polishing or CMP. The process has been widely used on ILD or IMD layers in fabricating modern semiconductor devices. A CMP process is performed by using a rotating platen in combination with a pneumatically actuated polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica or aluminum.
A polishing pad used on a rotating platen is typically constructed in two layers overlying a platen with a resilient layer as an outer layer of the pad. The layers are typically made of a polymeric material such as polyurethane and may include a filler for controlling the dimensional stability of the layers. A polishing pad is typically made several times the diameter of a wafer in a conventional rotary CMP, while the wafer is kept off-center on the pad in order to prevent polishing a non-planar surface onto the wafer. The wafer itself is also rotated during the polishing process to prevent polishing a tapered profile onto the wafer surface. The axis or rotation of the wafer and the axis of rotation of the pad are deliberately not collinear, however, the two axes must be parallel. It is known that uniformity in wafer polishing by a CMP process is a function of pressure, velocity and concentration of the slurry used.
A CMP process is frequently used in the planarization of an ILD or IMD layer on a semiconductor device. Such layers are typically formed of a dielectric material. A most popular dielectric material for such usage is silicon oxide. In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed is normally between about 5000 Å and about 10,000 Å. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window etching or plug formation difficulties. The CMP process has also been applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing.
The important component needed in a CMP process is an automated rotating polishing platen and a wafer holder, which both exert a pressure on the wafer and rotate the wafer independently of the rotation of the platen. The polishing or the removal of surface layers is accomplished by a polishing slurry consisting mainly of colloidal silica suspended in deionized water or KOH solution. The slurry is frequently fed by an automatic slurry feeding system in order to ensure the uniform wetting of the polishing pad and the proper delivery and recovery of the slurry. For a high volume wafer fabrication process, automated wafer loading/unloading and a cassette handler are also included in a CMP apparatus.
As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydrogenation reaction; the formation of hydrogen bonds between the wafer and the slurry; the formation of molecular bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP polishing process is not a mechanical abrasion process of slurry against a wafer surface.
While the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in controlling polishing rates and different locations on a wafer surface. Since the polishing rate applied to a wafer surface is generally proportional to the relative velocity of the polishing pad, the polishing rate at a specific point on the wafer surface depends on the distance from the axis of rotation. In other words, the polishing rate obtained at the edge portion of the wafer that is closest to the rotational axis of the polishing pad is less than the polishing rate obtained at the opposite edge of the wafer. Even though this is compensated by rotating the wafer surface during the polishing process such that a uniform average polishing rate can be obtained, the wafer surface, in general, is exposed to a variable polishing rate during the CMP process.
More recently, a new chemical mechanical polishing method has been developed in which the polishing pad is not moved in a rotational manner but instead, in a linear manner. It is therefor named as a linear chemical mechanical polishing process in which a polishing pad is moved in a linear manner in relation to a rotating wafer surface. The linear polishing method affords a uniform polishing rate across a wafer surface throughout a planarization process for uniformly removing a film player of the surface of a wafer. One added advantage of the linear CMP system is the simpler construction of the apparatus and therefore not only reducing the cost of the apparatus but also reduces the floor space required in a clean room environment.
A typical linear CMP apparatus 10 is shown in FIGS. 1A and 1B. The linear CMP apparatus 10 is utilized for polishing a semiconductor wafer 24, i.e. a silicon wafer for removing a film layer of either an insulating material or a wafer from the wafer surface. For instance, the film layer to be removed may include insulating materials such as silicon oxide, silicon nitrite or spin-on-glass material or a metal layer such as aluminum, copper or tungsten. Various other materials such as metal alloys or semi-conducting materials such as polysilicon may also be removed.
As shown in FIGS. 1A and 1B, the wafer 24 is mounted on a rotating platform, or wafer holder 18 which rotates at a predetermined speed. The major difference between the linear polisher 10 and a conventional CMP is that a continuous, or endless belt 12 is utilized instead of a rotating polishing pad. The belt 12 moves in a linear manner in respect to the rotational surface of the wafer 24. The linear belt 12 is mounted in a continuous manner over a pair of rollers 14 which are, in turn, driven by a motor means (not shown) at a pre-determined rotational speed. The rotational motion of the rollers 14 is transformed into a linear motion 26 in respect to the surface of the wafer 24. This is shown in FIG. 1B.
In the linear polisher 10, a polishing pad 30 is adhesively joined to the continuous belt 12 on its outer surface that faces the wafer 24. A polishing assembly 40 is thus formed by the continuous belt 12 and the polishing pad 30 glued thereto. As shown in FIG. 1A, a plurality of polishing pads 30 are utilized which are frequently supplied in rectangular-shaped pieces with a pressure sensitive layer coated on the back side.
The wafer platform 18 and the wafer 24 forms an assembly of a wafer carrier 28. The wafer 24 is normally held in position by a mechanical retainer, commonly known as a retaining ring 16, as shown in FIG. 1B. The major function of the retaining ring 16 is to fix the wafer in position in the wafer carrier 28 during the linear polishing process and thus preventing the wafer from moving horizontally as wafer 24 contacts the polishing pad 30. The wafer carrier 28 is normally operated in a rotational mode such that a more uniform polishing on wafer 24 can be achieved. To further improve the uniformity of linear polishing, a support housing 32 is utilized to provide support to support platen 22 during a polishing process. The support platen 22 provides a supporting platform for the underside of the continuous belt 12 to ensure that the polishing pad 30 makes sufficient contact with the surface of wafer 24 in order to achieve more uniform removal in the surface layer. Typically, the wafer carrier 28 is pressed downwardly against the continuous belt 12 and the polishing pad 30 at a predetermined force such that a suitable polishing rate on the surface of wafer 24 can be obtained. A desirable polishing rate on the wafer surface can therefore by obtained by suitably adjusting forces on the support housing 32, the wafer carrier 28, and the linear speed 26 of the polishing pad 30. A slurry dispenser 20 is further utilized to dispense a slurry solution 34.
In the conventional linear polisher 10, the polishing pads 30 are joined to the continuous belt 12 by adhesive means such as by a pressure sensitive. In a typical linear polisher, since the continuous belt 12 may have a length of about 240 cm, while the polishing pads 30 cannot be supplied in the form of a continuous manner, many pieces of the polishing pads 30 must be used. In other words, seam lines between adjacent polishing pads 30 must be formed when joined to the continuous belt 12. For instance, when the polishing pads are supplied in length of only about 30˜40 cm, between five and seven pieces of the polishing pads must be utilized.
The linear chemical mechanical polishing method provides the advantages of a high belt speed, a low compression force on the sample and the flexibility of using either a hard pad or a soft pad. However, a good planarity is achieved by the linear CMP method at the expense of polishing uniformity across a wafer surface. The poor polishing uniformity across the wafer surface is caused by the pattern of voids or protrusions utilized on a polishing pad in linear CMP. This is shown in FIGS. 2A, 2B and 2C.
As shown in FIG. 2A, the polishing pad 30 may be formed of a pad body 42 which has a top surface 44 and a bottom surface 46. It should be noted that in FIG. 2A, only a section of a polishing pad 30 is shown. The top surface 44 of the pad body 42 is provided with a multiplicity of grooves 50, each having a predetermined width and depth. The depth of the grooves 50 is normally smaller than the thickness of the pad body 42. FIG. 2B shows a similar pad body 42 but is provided with a multiplicity of apertures 52, i.e. perforations through the pad body 42. The diameter of each aperture in the multiplicity of apertures 52 is essentially the same. In another configuration, as shown in FIG. 2C, the pad body 42 is provided with a multiplicity of protrusions 54, i.e. mesas with grooves 56 therein between. The multiplicity of protrusions 54 are formed to a pre-determined thickness, i.e. to less than 3 mm. The top surface 58 of the multiplicity of protrusions 54 contacts a wafer surface during the linear CMP process.
In the conventional polishing pad 30 shown in FIGS. 2A, 2B and 2C, the polishing rate on a wafer surface contacting the top surface 44 of the pad is different across the wafer surface. For instance, it was found that at near the center line 60 of the pad body 42, polishing rate obtained is lower than the polishing rate obtained at two edge portions 62,64. The varying polishing rates across the width of the polishing pad 30 therefore causes poor uniformity on a wafer surface being polished. When the surface grooves 52 are of the same width (FIG. 2A), the apertures 52 are of the same diameter (FIG. 2B), or when the protrusions 54 are of the same size (FIG. 2C), poor uniformity in the thickness removed from the wafer surface is observed.
It is therefore an object of the present invention to provide a linear chemical mechanical polishing apparatus that does not have the drawbacks or shortcomings of a conventional linear CMP apparatus.
It is another object of the present invention to provide a linear CMP apparatus for achieving improved polishing uniformity on a wafer surface.
It is a further object of the present invention to provide a linear CMP apparatus for achieving improved planarity and uniformity on a wafer surface.
It is another further object of the invention to provide a linear CMP apparatus that is equipped with a programmable pneumatic support plate for supporting a polishing pad and achieving improved polishing uniformity on a wafer surface.
It is still another object of the present invention to provide a linear CMP apparatus equipped with a pneumatic support platen for the polishing pad that is divided into at least three separate zones each controlling a plurality of openings arranged in concentric circles.
It is yet another object of the present invention to provide a linear CMP apparatus equipped with a pneumatic support platen wherein a plurality of pneumatic zones is controlled by a plurality of pressure detectors, a plurality of flow regulators and a process controller.
It is still another further object of the present invention to provide a method for controlling the polishing profile on a wafer surface during a linear CMP process by flowing a gas through a plurality of apertures in a support platen for the polishing pad and forcing an intimate contact between a wafer surface and the polishing pad.
It is yet another further object of the present invention to provide a method for controlling the polishing profile of a wafer surface during linear CMP by a plurality of pressure detectors, a plurality of flow regulators and a process controller for controlling a pneumatic force exerted by a support platen.